Valid & Well-Formed XBRL

XBRL Instance Documents must be Well-Formed and Valid prior to filing with the SEC, but what is a Well-Formed XBRL Instance Document and what is a Valid XBRL Instance Document?

There are four general ideas that would describe a Well-Formed document.

The start and end tags match

Elements are properly nested

All of the elements are contained in the root element

Element names and attributes are defined in the XBRL Schema

A document that is Valid is by definition Well-Formed, but a Well-Formed document is not necessarily Valid. A document is Valid if it is both Well-Formed and all of its elements are contained in the XBRL Schema/Taxonomy.